Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic scheme.
Description of the Related Art
Developing assemblies in which a developer feed/stripping member is omitted have been proposed in order to reduce the size and lower the cost of developing assemblies of image forming apparatuses. Developer feed/stripping members have the function of supplying and stripping a developer (toner) to/from a developing roller (developer carrier). Developer feed/stripping members are used mainly as countermeasures against ghosting, solid image tracking defects and toner melt adhesion.
Herein, ghosting denotes a phenomenon where, upon formation of a halftone image following formation of a solid image of high density, vestiges of the solid image are visible on the halftone image. Solid image tracking defects are a phenomenon where the density at the trailing end of the image drops, and which occur when a 100% solid image is drawn over the entirety of the image. Toner melt adhesion is a phenomenon where every time that an image formation operation is performed, the toner on the developing roller is not replaced but remains therefor a long time. This is exacerbated with repeated rubbing, and, eventually, the toner becomes fixed to the surface of the developing roller. In order to omit a developer feed/stripping member, therefore, the above problems need to be dealt with by resorting to some other means.
Accordingly, developing assemblies from which a developer feed/stripping member has been scrapped have been proposed wherein the surface of the developing roller is configured to have dielectric portions and conductor portions that are distributed, mixed with each other regularly or irregularly, over the surface of the developing roller (Japanese Patent Nos. 3272056 and 3162219). Specifically, a developer regulating member rubs the dielectric portions of the developing roller surface, directly or via toner, so that, as a result, the dielectric portions become charged, and small closed electric fields are formed on portions, of the dielectric portions, adjacent to the conductor portions. Toner having been transported onto the developing roller surface is acted upon by gradient forces derived from the small closed electric fields. The toner is drawn thereby onto the developing roller surface, to be carried thereon.
In toner supply by gradient forces, however, the attraction force of the developing roller towards the toner may in some instances be stronger than in cases where a developer feed/stripping member is used, and toner melt adhesion may occur.
Technologies proposed for suppressing occurrence of toner melt adhesion in developing rollers that exploit gradient forces include methods that involve forming a predetermined potential difference during non-image formation, to cause thereby toner on the developing roller to migrate to a developer feed/stripping member (Japanese Patent Application Publication No. H09-197803). Specifically, schemes have been disclosed that involve forming a potential difference such that toner on the developing roller is attracted towards the developer feed/stripping member, to electrically strip off the toner on the developing roller.
In such a technology, however, a developer feed/stripping member has to be provided in order to electrically attract toner on the developing roller. This constitutes an impediment to reductions in size and cost.